1. Field of the Invention
The present invention relates to an optical system for an optical recording and reproducing system, and more particularly, to an optical system for an optical recording and reproducing system that is capable of reducing a thickness and volume of a lens and of performing a ultra-high density information recording.
2. Description of the Background Art
In order for an optical recording medium or an optical magnetic recording medium to have a high density recording capacity, a bit (or a record mark) size should be small and a track width should become narrow.
However, since the spot size of a light focussed on a recording medium to form a bit on a record film of the recording medium is restricted by a diffraction limitation, it has a limitation to improvement of a recording density.
According to the tendency that a large amount of information is to be stored, a fresh optical recording/reproducing method is requested to overcome the limitation of the existing optical recording/reproducing method.
Recently, researches are being increasingly conducted on a near field recording/reproducing using a near field, because it is anticipated to remarkably enhance the recording capacity.
The near field optical recording and reproducing is based on the following principle.
As for a light made incident into a lens with an angle more than a critical angle, when it proceeds from a portion where a refractivity is dense to a portion where a refractivity is coarse, the light is totally reflected.
At this time, owing to the total reflection, an infinitesimal strength of light exists on the surface of the lens, which is called an evanescent wave or a dissipation wave. Use of the evanescent wave enables a high resolution which has been not possible due to an absolution limitation of the resolution, that is, a diffraction limitation, caused due to a diffraction phenomenon of light in the existing far-field.
In the near field optical recording and reproducing optical system, light is fully reflected within a lens to generate an evanescent wave on the surface of the lens, and a recording and reproducing is performed by a coupling of the evanescent wave and the recording medium.
FIG. 1 is a perspective view showing a near field optical recording system 10.
A central portion of a disk 11, a recording medium, is mounted to be rotatable at a spindle motor (not shown) in a deck 18, and a recording and reproducing unit is installed at the portion therein.
At an upper surface of the disk a flying head slider 12 including a lens (19) thereon is supported by a suspension arm 14, and one side of the suspension arm is connected to a pick-up unit 17.
At a lower portion of the pick-up unit, a voice coil motor (VCM) 16 is installed, so that the pick-up unit can rotated at a certain range of angle.
Meanwhile, a fixing arm 13 is installed to be supported by the pick-up unit, and a prism 15 is installed at the end of the fixing arm.
Light generated from a light source (not shown) of the pick-up unit is changed in its path at the prism to pass the lens (not shown) mounted at the head slider and is finally made incident on the surface of the disk.
According to interaction between the incident light and the surface of the disk, light information can be recorded or reproduced.
FIG. 2 is a schematic enlarged view showing the optical system mounted at the head slider in the system of FIG. 1.
The optical system includes a hemispheric solid immersion lens (SIL) 22 and a first condensing lens 21.
The SIL is formed hemispheric shape of which an upper portion is spherical and a lower surface is plane, and it is installed so that the center of the plane portion of the SIL corresponds to a focus of the first condensing lens.
Accordingly, the light 24 made incident on the first condensing lens is refracted to be collected at the center of the lower plane portion of the SIL.
In order to record a data (a bit) on the disk by using the SIL, as shown in FIG. 2, the SIL comes near the surface of the recording medium 23 with very small space, that is, for example, with a space of 10˜70 nm.
Then, an optical near field phenomenon takes place that a portion of the optical energy first focussed at the lower surface of the SIL is transferred to the recording medium. Thanks to the near field phenomenon, it is possible to record or reproduce a data in or from the surface of the recording medium.
For example, the energy transferred from the SIL heats a portion of the surface of the recording medium, causing a local phase change. Bits are formed on the surface of the recording medium owing to the phase change. That is, information is recorded.
In case of reading information, a characteristic that a refractivity differs at the portion where the phase change has been made locally is used. A light having a lower strength than that of a light for recording is made incident through the SIL, and then the strength of the light coming through the SIL after being reflected on the surface of the recording medium is measured by means of an optical sensor. Then, since the refractivity differs according to existence and non-existence of bit, information can be read.
Though the conventional optical system using the SIL has an advantage that a diffraction limitation of light is overcome and light spot can be reduced, it also has the following problems.
That is, generally, an aberration that a light is not focussed to one point occurs in the optical lens. The aberration has a characteristic that it becomes greater as the magnification of a lens is higher.
Since the optical system using the SIL requires a first condensing lens of a large magnification, the aberration of the first condensing lens much degrades a first condensing performance of the optical system.
In addition, since the data recording and reproducing lens using the SIL requires the first condensing lens, the device is enlarged in its volume and complicated, and it is difficult to assemble an entire data storing unit and the first condensing lens.
Especially, since there is a limitation to reducing the height of a head slider where the lens is mounted, it is difficult to manufacture an ultra-thin optical recording and reproducing system that can be mounted at a mobile instrument.